The present invention relates to a photosensitive lithographic printing plate having thereon a microscopic embossment which is termed xe2x80x9cmatxe2x80x9d.
JP-A-58-137469/1983 (the term xe2x80x9cJP-Axe2x80x9d as used herein means an unexamined published Japanese patent application) deals with a process for providing mats on a recording material. It discloses the distribution (distribution number), the height and the size (diameter) of mats to be formed. JP-A-10-133383/1998 touches on a photosensitive lithographic printing plate provided with microscopic patterns. It describes the average diameter, the average height and the distribution (distribution number) of the macroscopic patterns to be formed.
However, various problems have been encountered in the course of investigations toward the present invention. Namely, pressure is applied to the mat-attached face of the photosensitive lithographic printing plate provided with mats resulting from the aforementioned prior art after production but before use (e.g., during storage, packaging and delivery etc.) so that the height of the mats will be varied to cause a problem that the effect of shortening vacuum adhesion time, which is the primary function of the mats, will be prominently deteriorated.
Accordingly, an object of the present invention is to provide a photosensitive lithographic printing plate in which prominent deterioration of the vacuum-adhesion-time-shortening effect which is a properly expected function of the mats will be prevented even in case that the matted face of a photosensitive lithographic printing plate having the mats thereon is subjected to pressure after production but before use of the photosensitive lithographic printing plate.
In order to solve the above problem, it would be one idea to attach a large amount of large-diameter mats for overcoming the applied pressure by virtue of their sufficient sum of areas. However, it was difficult to put this idea into practice because the mats of the large-diameter tend to deteriorate the reproducibility of small dots of small dot images upon exposure.
Therefore, the present inventors tried to control the mean volume of the mats and form many of the mats as largely as possible within a range that deterioration of small dot image reproducibility will be avoided. Consequently, it was found that a photosensitive lithographic printing plate provided with a specific ratio of the mats each having particular range of volume to the total mats can ensure good reproducibility of small dots of the small dot images, endure the pressure affecting after production but before use, and maintain the function of the mats. Thus the present invention has been completed on the basis of the above findings.
Namely, according to one aspect of the present invention, the above object can be attained by a photosensitive lithographic printing plate having mats attached thereon wherein a number of the mats, each having volume of not less than 4,500 xcexcm3 and less than 18,000 xcexcm3, exceeds 20% relative to a total number of the attached mats. The photosensitive lithographic printing plate of the present invention will be embodied as follows.
It is preferred to control the number of the mats each having volume of not less than 18,000 xcexcm3 to be less than 10% relative to the number of all the attached mats. The mats can be formed through atomizing a liquid containing resin dissolved therein with a rotary atomizer. All of the number ranges disclosed in this specification must include the both end values and also all of arbitrary midway values involved therebetween.
In the photosensitive lithographic printing plate of the present invention, the number of the mats each having volume of not less than 4,500 xcexcm3 but less than 18,000 xcexcm3 exceeds 20% relative to the total number of the mats attached thereon. Preferably, they are controlled to be not less than 30%, and more preferably, not less than 40%. The mats each having volume of not less than 18,000 xcexcm3 are preferably controlled to be less than 10% (more preferably, less than 8%) relative to the number of all the attached mats.
A photosensitive lithographic printing plate includes at least a substrate and a photosensitive layer, and optionally, other layers. Materials of the substrate and the photosensitive layer may be those conventionally used for the usual photosensitive lithographic printing plate. The mats may be provided on places where the vacuum-adhesion-time-shortening effect of their essential function can be taken. Usually, they are provided at least on the surface of the photosensitive layer.
The mats are preferably distributed not less than 25/mm2 (more preferably, not less than 30/mm2, still more preferably, not less than 40/mm2). The mats may be made of a material which has been conventionally used for the usual photosensitive lithographic printing plate such as various types of resin and etc.
[The Maximum Volume Value of the Mat]
A film image can be transferred to a photosensitive lithographic printing plate by bringing the film into close contact with the printing plate during exposure. When a thick air gap exists between the film and the photosensitive lithographic printing plate during this procedure, what is called xe2x80x9cburn blurxe2x80x9d phenomenon will occur, and accordingly, the image cannot be transferred faithfully. In order to avoid this, it is usually practiced that the film and the photosensitive lithographic printing plate are placed in a depressurized place to evacuate the air existed between them. A mat layer is necessary to be formed on the surface of the photosensitive lithographic printing plate for evacuating this air easily.
The mat layer has a function of maintaining a suitable gap between the film and the photosensitive lithographic printing plate which is appropriate for making the air existed between them easily evacuated. However, if the mat is too high, the following phenomenon will occur, i.e., the above gap will be enlarged too wide to make it impossible to transfer the film image to the printing plate with good fidelity.
Further, a large-diameter mat makes it difficult to transfer the image onto a mat-deposited spot. This phenomenon will occur as a result of excessively forming the mats with large volumes. Accordingly, it is important to inhibit the mat from exceeding certain volume. On this account, it is preferable to make the volume of one mat less than 18,000 xcexcm3.
[The Minimum Volume Value of the Mat]
A photosensitive lithographic printing plate can be prepared by forming a photosensitive layer, a mat layer and optionally other layer(s) on a continuous web-formed substrate. In this process, contact with rollers made of various materials, winding up, stacking after cutting, and other operations are performed. The prepared photosensitive lithographic printing plate reaches the user""s hand by way of preservation, packaging, delivery and other steps. In the periods of these steps, i.e., after production but before use, forces such as pressure and the like applied, for example, on the surface of the printing plate deform the shape of the mats formed on the surface. To cope with these forces, forming a large number of mats as large as possible is an effective way for decreasing the deformation of the mats. Accordingly, it is preferable to make the volume of one mat not less than 4,500 xcexcm3.
[Production of the Photosensitive Lithographic Printing Plate]
The photosensitive lithographic printing plate of the present invention can be prepared through forming mats, which include those of specific volumes as defined in the present invention which are in particular ratios as determined in the present invention, on a photosensitive lithographic printing plate or a precursor thereof. The finished or unfinished photosensitive lithographic printing plate can be prepared through forming, for example, at least a photosensitive layer on a substrate before forming mats. Materials of the substrate may be those conventionally used. The photosensitive layer may be made of materials conventionally used for that of the usual photosensitive lithographic printing plate and can be formed on a substrate by applying a conventional method.
A photosensitive lithographic printing plate is prepared by laminating photosensitive layer, mat layer and if necessary other layer(s) on a continuous web substrate running by means of rollers. After forming the mats, operations such as contact with rollers which may be made of any of various materials, winding up, and stacking (in layers) of cut sheets are performed, so that pressure is inevitably applied to the mats formed on the photosensitive lithographic printing plate. In case of preparing the photosensitive lithographic printing plate defined in the present invention, the deformation of the mats resulting from the pressure applied in the preparing process after forming the mats can be prevented.
[Formation of the Mats]
In the photosensitive lithographic printing plate of the present invention, mats can be formed on the surface thereof by attaching fine particles obtained by atomizing a liquid for the mats on a photosensitive lithographic printing plate (generally on the photosensitive layer""s surface thereof) and drying the fine particles attached to the photosensitive lithographic printing plate with or without heating. Here, the term xe2x80x9cliquid for the matsxe2x80x9d means a liquid containing a mat material for forming mats which involves both of the following cases, i.e., the case of dissolving the mat material in a solvent and the case of not dissolving but dispersing it in a dispersant.
As the mat materials, i.e., materials for forming the mats, materials conventionally used for the mats of the usual photosensitive lithographic printing plate such as various kind of resins and the like can be employed. Such mat materials include, for example, those disclosed in JP-A-57-34558/1982 such as copolymer of acrylate and acrylic or methacrylic acid; coplymer of styrene, acrylate and acrylic or methacrylic acid; coplymer of acrylate, styrene, acrylonitrile and the like with acrylic, methacrylic, maleic, itaconic or other acid; and vinyl polymers such as polyvinyl alcohol, polyvinyl acetate, polyvinyl pyrrolidone and the like.
In case of changing the mat material into the form of the liquid for the mats, it is preferable to prepare an aqueous liquid of dissolving or dispersing an appropriately selected resin by applying the conventional method. The solvent or dispersant of the mats includes water which may further contain organic solvents.
The liquid for the mats may further includes fillers, if they don""t exert a harmful influence, on the photosensitive layer or on the droplets of the liquid attached thereto, such as any other water-soluble substance, fine inorganic particle or powder, polymeric powder and the like.
An exemplary process for preparing the mats may include emulsion-polymerizing raw material monomers emulsified in water with a surfactant by using a polymerization initiator such as potassium persulfate and the like to form an aqueous dispersion in the same way as to synthesize usual latexes, or changing partially acrylic, methacrylic, maleic, itaconic and other acids into sodium, potassium or ammonium salt thereof to form a copolymer solution.
In order to prepare the mats each having the specific volume ranges as defined in the present invention into the particular ratio relative to the total mats as defined in the present invention, it is necessary to control the mean volume of the mats and also to reduce the fluctuation in the volume.
In case of applying electrostatic coating wherein the atomized liquid for the mats is attached to the desired portion by atomizing the liquid for the mats through the steps of feeding the liquid continuously into the head (hereinafter referred to xe2x80x9cbellxe2x80x9d) of a cup-shaped rotary atomizer and then charging the atomized liquid with electric charge, the size of the atomized liquid can be changed by determining appropriately the rotation of the bell, the shape of the bell and the feed of the liquid. Accordingly, the rotation of the bell, the shape of the bell and the feed of the liquid are controlled so as to form the mats including those having specific range of volumes defined in the present invention which are in a particular ratio as defined in the present invention.
The droplets (fine particles) of the liquid for the mats ever has a distribution of certain range in size. Thereby, a completely definite size of the fine particles cannot be formed. However, it is possible to form the fine particles having diameters of comparative narrow distribution under some condition by controlling the rotation of the bell, the shape of the bell and the feed of the liquid. Further, it is also possible to regulate the average diameter of the minute particles by bringing the above factors (the rotation of the bell, the shape of the bell and the feed of the liquid) under control.
Accordingly, selection of adequate conditions regarding the above factors makes it possible to set an average diameter of the fine particles to be formed close to a desired value and also to narrow the diameter distribution, which follows reducing a ratio of particles which are outside of the desired diameter range. The fine droplets of the liquid prepared by the above process generally attaches to the surface of the photosensitive layer included in the photosensitive lithographic printing plate and changes into the mats through the steps of drying and adhering thereto. Accordingly, it is possible to regulate the volumes of the mats to be formed by controlling the droplet sizes of the liquid properly.